Electrostatic printing upon an image recording medium comprises the formation of a latent, electrostatic image by the creation of air ions and the deposition of those of a given sign (usually negative) at selected image pixel locations on the recording medium. Subsequently, the electrostatic latent image is made visible by "toning", which usually involves the passing of the recording medium, bearing the latent (non-visible) image, into contact with a liquid solution containing positively charged dye particles in a colloidal suspension. The dye particles will be attracted to the negative charge pattern and the density of the dyed image will be an increasing function of the potential or charge on the recording medium.
Two types of image recording media in common usage are paper and film. The paper has a conductive bulk and a thin dielectric coating upon its image bearing side. The film comprises a dielectric substrate (such as Mylar.RTM.), a very thin intermediate conductive layer and a dielectric overcoat layer upon its image bearing side. Conductive edge stripes passing through a dielectric surface layer to the conductive layer provide electrical paths to the conductive layer. In the case of paper, the writing potential established in the conductive layer is obtained by contact, i.e. by a combination of resistive and capacitive coupling, and in the case of film, the potential established in the conductive layer is obtained by capacitive coupling through the dielectric substrate.
Conventionally, an electrostatic image may be formed upon a recording medium, such as paper, having a thin surface dielectric layer coated upon a conductive base material. The recording medium is passed between a recording head, including an array of recording stylus electrodes, and a counter electrode comprising an array of complementary backplate electrode segments. A charge is applied to the recording medium through a pair of coincident voltage pulses applied to opposite surfaces of the medium by the stylus electrodes and the backplate electrodes. When the potential difference between the stylus electrodes and the recording medium conductive layer rises enough to cause the voltage in the air gap therebetween to exceed the breakdown threshold of the air, the air gap becomes ionized and air ions of the opposite sign to the potential of the conductive layer are attracted to the surface of the dielectric layer. As the dielectric surface charges up, there is a corresponding drop in voltage across the gap, so that when the voltage across the gap drops below the maintenance voltage of the discharge, the discharge extinguishes, leaving the dielectric surface charged. The discharge potential of several hundred volts may be established by applying a voltage of a first polarity, e.g. on the order of -300 volts, to the stylus electrodes contemporaneously with the application of a substantially equal voltage of the opposite polarity, e.g. +300 volts, to the backplate electrodes. This causes the electrical discharge, imposing a localized negative charge to the surface of the dielectric layer of the recording medium.
Electrostatic recorders may be typically from 11 inches to 44 inches wide, and in some cases even as wide as 72 inches. Therefore, the writing head stylus array which extends fully across this width may have as many as 2000 to over 17,000 styli (at resolutions of 200 to 400 dots per inch). Because of this very large number of styli it is ordinarily not economically attractive to use a single driver per stylus, and a multiplexing arrangement is commonly used in conjunction with the above-described electrostatic discharge method. The styli in the writing head array are divided into stylus electrode groups (each group being about 0.5 inch to 1.5 inches long) so that each may consist of several hundred styli. Then the stylus electrodes are wired in parallel so that corresponding styli in each group, or every other group, are connected to a single driver and carry the same information. A selected stylus group writes only when its complementary electrode is pulsed.
In U.S. Pat. No. 4,424,522 (Lloyd et al) entitled "Capacitive Electrostatic Stylus Writing With Counter Electrodes" there is disclosed a counter electrode assembly of the backplate type which is conformable to the arcuate crown of the recording head. A structure of this type is illustrated in FIGS. 1, and is more fully described below. It comprises a plurality of parallel laminated segments of an insulating substrate overcoated with a resistive material, each segment is mounted upon an elongated, U-shaped, support bar so as to be electrically independent. The laminate material is stretched over the mouth of the support bar upon supporting legs and a resilient foam material is introduced into the channel of the support bar for urging the surface resistive material into intimate contact with the recording medium. In its commercial application, in electrostatic printer/plotters manufactured by the assignee of the present patent application, the channel of the support bar is provided with a strip of foam and an oil-filled bladder for urging the segmented backplate electrodes toward the writing head.
Use of the segmented counter electrode structure may result in striations, i.e. visible striping on the printed image extending in the direction of movement of the recording medium. One possible source of counter electrode caused striations is the large voltage gradients induced in the image recording medium between pulsed and non-pulsed electrodes. Another possible source is the gap between backplate electrode segments necessitated by mounting tolerances for preventing electrical shorting. The cutting process by which the segments are formed can also cause problems. When the cut is made with a stamp, die or knife edge, the edges tend to be frayed, resulting in an increased probability of shorting between the segments. When the cut is made by a laser, the melted edge tends to be carbonized and beaded or thickened and, during mounting and alignment of the segments upon the U-shaped, support bar, this carbonized bead can smear on the support bar causing a subsequent shorting path. Still another disadvantage of the segmented backplate electrode structure is that uniform wrapping tension of each segment upon the support bar is difficult to achieve, resulting at times in curling of the segment edges which can allow debris and chaff to collect in the gaps and to provide a shorting path. The non-uniform tension can also result in differing image intensity across the plot and differing wear across the writing head.
It is the primary object of this invention to eliminate the segmentation of the backplate electrodes by forming the backplate electrode as a continuous structure. Another object is to improve the wear characteristics of the backplate electrode structure. A further object is to alter the voltage gradients across the pulsed electrodes and between pulsed and non-pulsed electrodes.